Income Taxes (Details)		12 Months Ended
		Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2015 USD ($) $ / kWh	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Tax Audits [Abstract]
Tax Adjustments, Settlements, and Unusual Provisions		$ (5,000,000)	$ (17,000,000)	$ (12,000,000)	$ (15,000,000)
Consolidated Appropriations Act of 2016 [Abstract]
Excise Tax Delay		2 years
Tax Increase Prevention Act of 2014 [Abstract]
Number of years bonus depreciation was extended			1 year	1 year
American Taxpayer Relief Act of 2012 [Abstract]
Original top tax rate for dividends								15.00%
New top tax rate for dividends								20.00%
Federal Tax Loss Carryback Claims [Abstract]
Number Of Years Of Tax Loss Carryback Period		2
Tax Adjustments, Settlements, and Unusual Provisions		$ (5,000,000)	$ (17,000,000)	$ (12,000,000)	(15,000,000)
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions						$ 25,800,000	$ 16,200,000
Unrecognized tax benefit — Temporary tax positions						94,900,000	50,300,000
Total unrecognized tax benefit		66,500,000	41,200,000	34,500,000	34,500,000	120,700,000	66,500,000	$ 41,200,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1		66,500,000	41,200,000	34,500,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions		27,100,000	28,700,000	15,100,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions		(4,500,000)	(2,000,000)	(400,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions		34,800,000	16,000,000	21,600,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions		(2,900,000)	(6,000,000)	(4,800,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		(300,000)	(9,600,000)	(24,800,000)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations		0	(1,800,000)	0
Balance at Dec. 31		$ 120,700,000	$ 66,500,000	$ 41,200,000	$ 34,500,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards						(36,700,000)	(28,500,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible						58,000,000
Amounts accrued for penalties related to unrecognized tax benefits						0	0	$ 0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		35.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		4.10%	4.00%	4.10%
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits, Percent		0.60%	0.20%	0.60%
Effective Income Tax Reconciliation, Net operating loss carryback, Percent		(0.30%)	(0.90%)	(0.80%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items, Percent		(1.00%)	(1.30%)	(1.60%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent		(2.70%)	(2.60%)	(2.60%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(0.20%)	(0.50%)	(0.90%)
Effective Income Tax Rate Reconciliation, Percent		35.50%	33.90%	33.80%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)		$ (36,129,000)	$ (73,160,000)	$ (46,173,000)
Current State and Local Tax Expense (Benefit)		2,324,000	9,225,000	7,678,000
Current Change In Unrecognized Tax Expense (Benefit)		45,933,000	23,915,000	13,162,000
Deferred Federal Income Tax Expense (Benefit)		480,078,000	505,236,000	439,085,000
Deferred State and Local Income Tax Expense (Benefit)		92,132,000	84,787,000	80,907,000
Deferred Change In Unrecognized Tax Expense (Benefit)		(36,342,000)	(20,645,000)	(4,930,000)
Deferred investment tax credits		(5,277,000)	(5,543,000)	(5,753,000)
Income Tax Expense (Benefit)		542,719,000	523,815,000	483,976,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items		546,664,000	616,934,000	588,053,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities		(11,810,000)	(48,674,000)	(64,420,000)
Other Comprehensive Income (Loss), Tax		1,013,000	1,117,000	(8,572,000)
Deferred income tax expense (benefit), other		1,000	1,000	1,000
Deferred Income Tax Expense (Benefit)		$ 535,868,000	$ 569,378,000	$ 515,062,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment						7,119,023,000	6,257,191,000
Deferred Tax Liabilities, Regulatory Assets						313,414,000	300,762,000
Deferred Tax Liabilities, Other						243,690,000	300,251,000
Deferred Tax Liabilities, Gross						7,676,127,000	6,858,204,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards						851,242,000	552,274,000
Deferred Tax Assets Tax credit carryforward						404,738,000	346,064,000
Deferred Tax Assets Unbilled Revenue Fuel Costs						57,220,000	55,021,000
Deferred Tax Assets Rate Refund						50,441,000	93,956,000
Deferred Tax Assets Regulatory Liabilities						41,541,000	49,712,000
Deferred Tax Assets Environmental Remediation						38,663,000	42,716,000
Deferred Tax Assets Deferred Investment Tax Credits						29,650,000	31,886,000
Deferred Tax Assets, Valuation Allowance						(27,679,000)	(3,402,000)
Deferred Tax Assets, Other						76,869,000	83,199,000
Deferred Tax Assets, Net of Valuation Allowance						1,522,685,000	1,251,426,000
Deferred Tax Liabilities, Net						6,153,442,000	5,606,778,000
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Year(s) under examination		2012 and 2013			2010 and 2011
Year of carryback claim under examination		2009
Potential Tax Adjustments		$ 14,000,000
Operating Loss Carryforwards						2,153,000,000	1,349,000,000
Tax Credit Carryforward, Amount						360,000,000	327,000,000
Carryforward expiration date range, low		2021
Carryforward expiration date range, high		2035
Colorado
Tax Audits [Abstract]
Year(s) under examination		None
Earliest year subject to examination		2009
Minnesota
Tax Audits [Abstract]
Year(s) under examination		None
Earliest year subject to examination		2009
Texas
Tax Audits [Abstract]
Year(s) under examination		None
Earliest year subject to examination		2009
Wisconsin
Tax Audits [Abstract]
Earliest year subject to examination		2011
State and Local Jurisdiction
Tax Audits [Abstract]
Operating Loss Carryforwards						2,124,000,000	1,722,000,000
Operating Loss Carryforwards, Valuation Allowance						(65,000,000)	(53,000,000)
Tax Credit Carryforward Net Of Federal Detriment	[1]					45,000,000	19,000,000
Valuation Allowance for Tax Credit Carryforward Net of Federal Benefit	[1]					(24,000,000)	0
Federal detriment						24,000,000	$ 10,000,000
Federal Benefit						$ 13,000,000
Carryforward expiration date range, low		2016
Carryforward expiration date range, high		2035
Consolidated Appropriations Act of 2016; 2015, 2016, 2017 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent						50.00%
Consolidated Appropriations Act of 2016; 2018 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent						40.00%
Production Tax Credit Rate, Percent						60.00%
Consolidated Appropriations Act of 2016; 2019 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent						30.00%
Production Tax Credit Rate, Percent						40.00%
Investment Tax Credit Rate, Percent						30.00%
Consolidated Appropriations Act of 2016; 2020 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent						26.00%
Consolidated Appropriations Act of 2016; 2021 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent						22.00%
Consolidated Appropriations Act of 2016; After 2021 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Investment Tax Credit Rate, Percent						10.00%
Consolidated Appropriations Act of 2016; 2015 Impact [Domain]
Tax Audits [Abstract]
Federal detriment						$ 3,500,000
Federal Benefit						4,900,000
Consolidated Appropriations Act of 2016 [Abstract]
Other Deductions and Charges						1,200,000,000
Federal R&E Benefit						$ 6,800,000
Consolidated Appropriations Act of 2016; 2016 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent						100.00%
Production Tax Credit per KWh | $ / kWh						0.023
Consolidated Appropriations Act of 2016; 2017 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Production Tax Credit Rate, Percent						80.00%
Tax Increase Prevention Act of 2014; 2014 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent							50.00%
Tax Increase Prevention Act of 2014; 2015 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent						50.00%
American Taxpayer Relief Act of 2012; 2013 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent								50.00%
American Taxpayer Relief Act of 2012; 2014 Impact [Member]
Consolidated Appropriations Act of 2016 [Abstract]
Bonus depreciation rate, Percent							50.00%

[1] State tax credit carryforwards are net of federal detriment of $24 million and $10 million as of Dec. 31, 2015 and 2014